Machine readable codes, such as barcodes, QR codes, visual features or patterns, and watermarks, such as the Digimarc® barcode, are essentially representation of information in a visual format. Such codes may include data characters and/or overhead characters represented by a particular sequence of bars and/or spaces that may have varying widths. Such codes have widespread applications. For example, machine readable codes can be used to identify a class of objects or unique items. As a result, these codes are found on a wide variety of objects, such as documents, retail goods, shipping boxes, product parts, company assets, and so on.
There are several types of data readers used for reading machine readable codes. The most common types of readers are laser based barcode scanners and imaging based barcode scanners. A laser based barcode scanner typically moves, i.e. scans, a laser light beam across the barcode. Imaging based barcode scanners typically include solid state image circuitry, such as charge coupled devices (CCD) or complementary metal-oxide semiconductor (CMOS) devices, and may be implemented using a one-dimensional or two-dimensional imaging array of photo sensors or pixels to capture an image of the optical code. One-dimensional CCD readers capture a linear cross section of the code, producing an analog waveform whose amplitude represents the relative darkness and lightness of the code. Two-dimensional CCD or CMOS readers may capture an entire two-dimensional image.
Although the code readers may rely on ambient light when capturing an image, several imaging readers may utilize an illumination source to illuminate the optical code in an attempt to improve the image data quality generated by the imaging device as well as to provide a certain signal response in the imaging device. Such a source of illumination can reduce exposure time and thereby improving imager performance, especially in low ambient light conditions and when imaging moving items. The use of active illumination for scanners is to provide good contrast on an image sensor for readable optical indicia that include barcode labels, digital watermark labels, and visual features or patterns, etc., on objects and/or package surfaces; while minimizing visibility and distraction of the illumination to users.
Most barcode scanners use single color illumination consisting of a red light emitting diode (LED) as an illumination source. The single color illumination limits the range of colors that can be used in the printed barcode and be detected with the scanner. A similar problem also occurs when using such scanner to read digital watermarks. Thus, although the conventional barcode scanners typically use red illumination, some newer barcode scanners are increasingly moving to white LEDs as illumination for scanners as opposed to the traditional red LED illumination. One of the reasons behind this change is that red illumination can be more stressful on the eyes when used for long periods. Another reason is that red is more distracting because it does not blend in with the natural ambient light in the room, and hence there is less color contrast between the ambient and scanner illumination.
There are multiple ways to generate white light. One way is by mixing tri-chromatic lights, for example, red light, green light, and blue light (RGB lights). However, since RBG LED chip power ratings are similar for typical RGB LEDs, those LEDs are relatively expensive, while a green LED in the tri-color LED package can be eliminated functionally for scanner active illumination. Moreover, the respective wavelengths of red and blue (R and B) colors are not ideal for scanner applications. Another way that white light LEDs are typically produced is by coupling an LED of one color with a phosphor coating of a different color to produce white light. As illustrated in FIG. 2, a light source that appears to be white is constructed from a blue LED 202, such as an LED made from an indium gallium nitride semiconductor transmitting blue light 204, that is covered with a layer of phosphor 206 that converts a portion of the blue light to a broad band of yellow light 208. When the ratio of blue 204 to yellow light 208 is chosen correctly, the resultant light source appears white. The overall efficiency of the phosphor-converted light source is determined by the efficiency with which the LED converts electricity to light and the efficiency with which the LED light is down-converted to white light in the portion of the spectrum to which the eye is sensitive. When blue light is converted to white light, the difference in energy of photons is lost in the form of heat and therefore PC-LED has lower energy efficiency at a high power consumption. Furthermore, there is a great deal of watermark signal loss on monochrome image sensors due to PC-LEDs having a wide spectrum, as understood in the art. Therefore, such a white phosphor-based LED requires an optical filter that may be placed adjacent to the LED in order to improve watermark reflectance signal strength, and thereby allowing the white phosphor-based LED to better read watermarks.
As understood, the above-described conventional white phosphor-based LEDs that are currently available have low energy efficiency and high cost of production due to the requirement of additional components, such as an optical filter in order to read watermarks, etc. Therefore, there is a need for an improved white light illumination device and method of producing white light illumination for a machine readable indicia scanner, such as a barcode scanner, used to read barcodes watermarks, and visual features or patterns.